SCIENCE

Extreme summer heat -- like the sizzling temperatures that hit Texas and Oklahoma last year and Moscow in 2010 -- occurs far more frequently now than it did 30 years ago, according to a new study from NASA climatologist James Hansen.

Between 1951 and 1980, just 0.2 percent of Earth's land areas experienced that kind of scorching summer, on average. Today, that number has soared to 10 percent, according to Hansen's statistical analysis of global temperature data. And the likely cause is man-made climate change.

"We can state, with a high degree of confidence, that extreme anomalies such as those in Texas and Oklahoma in 2011 and Moscow in 2010 were a consequence of global warming because their likelihood in the absence of global warming was exceedingly small," Hansen and his co-authors wrote in a study that will be published this week in the Proceedings of the National Academy of Sciences.

The scientist put it more bluntly in an op-ed in yesterday's Washington Post: "The future is now. And it is hot."

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Hansen's study does not rely on climate models or attempt to link the change in heat wave frequency to carbon dioxide emissions. It is based on observed temperature trends -- a statistical analysis of the global temperature record maintained by NASA's Goddard Institute for Space Studies, which Hansen leads.

The researchers examined changes in summer temperatures -- averaged monthly for June, July and August -- in the Northern Hemisphere, comparing modern-day conditions with their 1951-1980 baseline period.

The obvious question may be the wrong one

The findings try to answer a question that seems to crop up every time extreme weather strikes: Is this climate change? Last year alone, the United States was hit with a record-breaking 14 natural disasters that each caused $1 billion or more of damage.

For many years, scientists often answered that question by noting that it is impossible to say with certainty whether climate change caused any particular weather event.

But researchers have since developed new methods of examining the link between warming and changes in the frequency and severity of weather trends. Many now answer such questions by explaining that climate change is changing the odds of many kinds of extreme weather.

It is a conclusion supported by a recent Intergovernmental Panel on Climate Change report that found evidence that climate change is increasing the frequency of drought and heat waves and the intensity of rainstorms, warning that such shifts will require the world's governments to change how they cope with natural disasters.

But to Hansen -- a scientist and outspoken advocate for policies to fight warming -- "Is it warming?" when a natural disaster strikes is posing the wrong question.

"Our analysis shows that it is no longer enough to say that global warming will increase the likelihood of extreme weather and to repeat the caveat that no individual weather event can be directly linked to climate change," he wrote in his Washington Post op-ed. "To the contrary, our analysis shows that, for the extreme hot weather of the recent past, there is virtually no explanation other than climate change."

Some criticize policy discussion

Kevin Trenberth, head of the Climate Analysis Section at the National Center for Atmospheric Research, said the new study "does a reasonably good job in demonstrating the changes in the distribution of [temperatures] over time."

While Hansen's study draws on previous research to infer a link between more frequent extreme heat and climate change, that is a reasonable choice, said Trenberth. He's co-author of a paper to be published in the Journal of Geophysical Research that examines how climate change is affecting global weather patterns and the role those changes played in the 2010 Russian heat wave, 2011 floods in Queensland, Australia, and other recent extreme weather events.

"Once you know global warming is present, why would you have to do it over and over again?" said Trenberth, adding later that "we're breaking [heat] records in a way that is clearly exceptional -- we're not breaking records the same way on the cold side."

Other scientists are not convinced that Hansen's study adds much to ongoing study of climate change's influence on weather extremes.

Martin Hoerling, a scientist at NOAA's Earth System Research Laboratory who has published several papers examining the role climate change played in individual weather events, like the 2010 Russian heat wave, said Hansen's analysis "does not present new science."

He criticized Hansen for including a brief discussion of policy responses to combat warming.

"This is obviously not meant to be a scientific paper, but it much closer to an extended op-ed piece," Hoerling said in an email. "The paper argues for a policy response, and supports the policy argument within an assessment of the recent global temperature record, and additional surmised implications of temperature on climate."